An independent federal privacy watchdog has concluded that the National Security Agency’s program to collect bulk phone call records has provided only “minimal” benefits in counterterrorism efforts, is illegal and should be shut down. The findings are laid out in a 238-page report, scheduled for release by Thursday and obtained by The New York Times, that represent the first major public statement by the Privacy and Civil Liberties Oversight Board, which Congress made an independent agency in 2007 and only recently became fully operational. The report is likely to inject a significant new voice into the debate over surveillance, underscoring that the issue was not settled by a high-profile speech President Obama gave last week. Mr. Obama consulted with the board, along with a separate review group that last month delivered its own report about surveillance policies.